Acrylate pressure-sensitive adhesives have been well known in the art. In U.S. Pat. No. 24,906 (Ulrich), alkyl acrylate copolymers are described which are predominately alkyl esters of acrylic acid having from 4 to 14 carbon atoms, and further comprise a minor amount (3-12%, preferably 4-8%) of a polar copolymerizable monomer such as acrylic acid. Such adhesives are widely popular as they are readily available and provide a good balance of tack, shear and peel properties on a variety of substrates at a relatively low cost.
These adhesives have a wide variety of uses in which high performance is required, including automotive, industrial and consumer applications.
U.S. Pat. No. 4,181,752 (Martens et al) discloses a process for ultraviolet photopolymerization of alkyl acrylate esters and polar copolymerizable monomers to form the acrylate copolymer. Martens teaches that intensity and spectral distribution of the irradiation must be controlled in order to attain desirable cohesive strengths and peel resistance. The photopolymerization is preferably carried out in an inert atmosphere as oxygen tends to inhibit the reaction. Adhesive properties for tapes made via the Martens process are improved over those made by solution polymerization.
Many fillers, including various types of microbubbles have been used in acrylate adhesives. U.S. Pat. No. 4,931,347 (Slovinsky et al.) discloses a pressure-sensitive adhesive matrix containing particulate solids wherein the adhesive system has a light transmission of 15-80 percent. The particles have an average size no greater than 300.mu.. Acrylic acid ester monomers are disclosed as preferred for the adhesive matrix. Preferred particles are polymers derived from ethylenically-unsaturated monomers. UV-initiated polymerization of the adhesive is disclosed.
U.S. Pat. No. 4,243,500, (Glennon), discloses a pressure-sensitive adhesive formed from a composition comprising at least one monofunctional unsaturated acrylate ester monomer, a saturated tackifying resin polymer, a non-crystallizing elastomeric material, and an initiator responsive to ultraviolet light or other radiation The intensity of the lamps to which the adhesive is exposed is much higher than that of the lamps disclosed in Martens et al. The elastomer used is soluble in the acrylate ester, and therefore cannot form an interpenetrating polymer network (IPN) within the system.
U.S. Pat. No. 4,223,067 (Levens) discloses an acrylic foam-like pressure-sensitive adhesive containing clear glass microbubbles. U.S. Pat. No. 4,612,242, (Vesley et al.), U.S. Pat. No. 4,748,061 (Vesley), U.S. Pat. No. 4,666,771, (Vesley et al.), also disclose acrylic pressure-sensitive adhesives containing various types of colored coated glass microbubbles which yield a dark pressure-sensitive foam-like adhesive. Levens and Vesley provide pressure-sensitive adhesives having low tack and which are translucent or opaque, requiring the use of pigments when it is desired to render the adhesive layer less visible when placed in contact with a painted surface. Furthermore, fillers and/or prepolymerization is necessary to provide coatable compositions, especially, when very thick constructions are desired.
Continuous IPN's are described in U.S. Pat. No. 4,302,553 (Frisch et al.). The IPNs are composed of chemically dissimilar crosslinking polymer chains having substantially no chemical bonding between dissimilar chemical chains, but rather, the dissimilar chains are physically entangled, as fibers in a cloth, by permanent chain entaglements. Utility as resins for the preparation of films, elastomers, adhesives, coatings, and the like is disclosed.
U.S. Pat. No. 3,833,404 (Sperling et al.) describes a damping means for a vibratory or noise-emitting part or surface comprising an inner layer or coating of viscoelastic material and an outer plastic constraining layer. The viscoelastic material is comprised of a continuous IPN consisting essentially of 5-95% by weight of crosslinked plastic and 95-5% of crosslinked elastomer.
U.S. Pat. No. 4,616,057 (Lindemann et al.) discloses a process for preparing polymer emulsions-containing IPNs. The polymer emulsions are useful as binders, adhesives, and coatings. When the polymer emulsion is subsequently applied, dried, and heated, the physical and chemical boding of the two polymer networks is completed.
U.S. Pat. Nos. 4,396,476 and 4,396,377 (Roemer et al.) and U.S. Pat. Nos. 4,551,486, 4,698,373 and 4,711,913 (Tateosian et al.) describe dental compositions hardenable by exposure to heat or electromagnetic radiation. The composition, in general comprise crosslinked polymer in the form of discrete particles, polymerizable monomer, and crosslinking agent. Crosslinking oligomers are sometimes utilized. A discontinuous IPN structure is thought to be formed and is superior grind resistance, of the hardened dental compositions.
U.S. Pat. No. 4,855,170, (Darvell et al.) discloses a sheet material coated with a layer of pressure-sensitive adhesive containing and completely surrounding resilient non-tacky hollow polymeric microspheres which is disclosed to yield a removable and repositionable product. However, no IPN is formed within the boundaries of the microspheres since the matrix does not extend through the microspheres themselves.
WO 89/00106 (Avery) discloses an extruded pressure-sensitive adhesive comprising a carrier layer comprising an electron beam cured pressure-sensitive adhesive matrix, 10-20% by volume low density microspheres, and at least one pigment. The curable matrix comprises one or more monomers which have been previously polymerized. The solvent mixture of polymer, microsphere, pigment, etc., is fed through an extruder such that less than about 2% by volume of volatiles remains in the extrudate. The extruded film is then cured by electron beam radiation. It is further disclosed that UV-radiation is not suitable for curing of the tape as pigment is contained in the matrix. The matrix does not extend through the micro-sphere walls therfore no IPN can be formed.
In view of the foregoing, it is an object of this invention to provide an acrylate pressure-sensitive adhesive which exhibits the high performance characteristics of filled systems while providing the advantages of optical transparency and excellent pressure-sensitive tack.
It is also an object of this invention to provide a composition, curable to an acrylate pressure-sensitive adhesive, having enhanced coating rheology without the need for a prepolymerization step, added polymer, or addition of other rheology modifiers.
It has been discovered that these and other objects and advantages which will become apparent from the description below can be achieved by preparing acrylate pressure-sensitive adhesives containing IPNs within the boundaries of dispersed, swellable, acrylate, elastomeric microspheres.